Benthogone rosea Koehler, 1895

Benthogone rosea Koehler, 1895 View in CoL

Benthogone rosea Koehler, 1896: 114–117 View in CoL , figs 2, 3, 36, 46. —

Perrier, 1902: 399–405, pls. 14: 1–2, 19: 8–14. — Grieg, 1921: 5–6. —

Hérouard, 1923: 38–39. — Heding, 1940: 369. — Madsen, 1947: 15–

16. — Pawson, 1965: 219–221, pl. 5. — Gebruk et al. 2014: 159. Benthogone rosea var. cylindrica Perrier, 1896: 900 . Benthogone rosea var. 4- lineata Perrier, 1896: 900. Benthogone quadrilineata .— Heding, 1940: 369. — Heding, 1942: 15. Non Benthogone quatrolineata .— Augustin, 1908.

Material examined. NMV F296864 About NMV * (12) [IN 2021 V 04 031] ; NMV F308147 About NMV (1) and NMV F310371 About NMV (1) [IN 2022 V 08 105] ; NMV F308320 About NMV (1) [IN 2022 V 08 187] .

Diagnosis of IOT material. Specimens typically gelatinous, light red, pink, or mauve colour before preservation, with ventral colour often darker, particularly along the midventral, which was occasionally furrowed. Firmer and purple to mauve when preserved. Elongate body slightly raised and curved dorsally, rounded anteriorly and posteriorly and with thin lateral edges. Anus terminal, subdorsal. Ventral mouth often with a darker colouration than body, 15–20 retractable peltate purple tentacles surrounded by circumoral ring of small papillae (fig. 12c). Dorsal radial rows of slender papillae (often retracted and/or hard to detect), and ventral retractable tube feet on lateral margins, with end-disks often visible. IOT specimens up to 110 mm long, 17 mm wide and 7 mm high (NMV F308320 preserved), but average specimen less flattened (e.g. 65 mm long, 12 mm wide and 12 mm high; NMV F296864.1 – preserved). Body wall ossicles large to small Laetmogone -like wheels, slightly curved, with central four-rayed raised primary cross. Wheelssymmetrical but perimeter is undulating and size graduates from small to large. Larger wheels more typically with eight spokes ~104– 218 μm dorsally but smaller ventrally. Smaller wheels nine (typically 10 or more) spokes and ~70–118 μm. No teeth on rims. The wheel centre is almost filled, with only a narrow slit or hole in the calcareous membrane (presumably the nave as noted in Hansen, 1975), typically below the primary cross. Tentacles with clumps of irregular, curved spinous rods plus wheels. Wheels also observed in papillae and tube feet.

Remarks. There are many similarities with Laetmogone , but for intact specimens the presence of circumoral papillae distinguishes Benthogone from other genera in Laetmogonidae . The IOT material was a good match to the type description for B. rosea , though the midventral was more rust-red than “striking purple” and some of the wheel sizes differed as noted below.Distinguished from B. fragilis , which has greater number and different distribution of papillae, and from B. abstrusa , which has unretractile larger tube feet and conical dorsal papillae. Spinous rods were only observed in tentacles, some of the larger wheel ossicles were up to 218 μm – much larger than the type specimens for the species in Koehler (1895) (100 μm) and those observed in Hansen (1975) (160 μm). Hansen also saw no correlation in wheel size and spoke number, whilst we observed (like Koehler in the type specimens) typically eight spokes in the larger wheels for IOT specimens, and typically ten or more spokes for smaller wheels.Some specimens had lost some or all their 15–20 tentacles though trawl damage. Note that the outer skin was susceptible to damage or stripping during collection, and specimens clumped together during preservation, making it harder to see external characters later. Good shipboard photography in and out of water pre-preservation assisted with the observation of external characters. In addition to the original description (and illustration) in Koehler (1895), Hansen (1975) provides some good descriptions and keys and can be used in conjunction with the Laetmogone character table from Thander (1998) to rule out Laetmogone and other Benthogone species. The specimen sequenced for COI and 16S (NMV F296864) does not group with Benthogone abstrusa in either gene tree; genetic placement within Elasipodida is unclear (fig. S1). Based on morphological features, we maintain its identification here as B. rosea .

Distribution. Eastern North Atlantic from Ireland to Cape Verde (off Mauritania), north of New Zealand (South Pacific), and Indian Ocean (off east Africa and Australian IOT) ( Hansen, 1975; Gebruk et al., 2014; this work).

Full bathymetric range. 1103–2480 m ( Hansen, 1975; Gebruk et al., 2014) (IOT 2189–2435 m).

Type locality. North Atlantic , Bay of Biscay, 1300 m .

This species not previously recorded from Australia in AFD or ALA (January 2024) .

This IOT material represents a geographic range extension for the Indian Ocean and first record for Australia.

References. AFD (2024), ALA (2024), Gebruk et al. (2014), Hansen (1975), Koehler (1895), Massin (1993), Thandar (1998).